The goal of this Project is to induce islet allograft tolerance in the absence of generalized immunosuppression. We believe that the optimal alternative to current islet transplantation strategies is the induction of immunological tolerance. Tolerance induction is not currently a clinical reality because its mechanisms are incompletely understood, but new discoveries have increased our understanding substantially. This Program Project is based on one of these new discoveries and will focus on two key activation molecules, CD40 and CD40 ligand (CD4OL). These molecules play a central role in T cell activation and tolerance induction. Understanding of the cellular (Projects $2, $3, and this Project), molecular (Project $4), and biochemical (Projects $3 and $4) activities of CD40 and CD40L should enable us to achieve islet graft tolerance without generalized immunosuppression. In this Project, we will induce immunological tolerance in both chemically diabetic recipients of islet allografts, and in allograft recipients whose diabetes is due to autoimmunity. Even syngeneic grafts are rejected by the latter. We will focus on the CD40-CD40L co-activation system based on our published observation that it Can be manipulated to induce tolerance to islet allografts in the absence of immunosuppression. We have demonstrated that a two element therapy comprised of allogeneic splcen cells and short term treatment with anti-CD40L mAb renders animals tolerant to islet and skin allografts obtained from the splcen cell donors. In preliminary adoptive transfer studies, we have demonstrated the allospecificity of the tolerant state induced by the two element procedure. The data strongly suggest that the protocol induces a state of antigen specific tolerance rather than systemic immunosuppression. We will first identify the optimal parameters for tolerance induction in both normal mice and in a novel T cell receptor (TcR) transgenic mouse (Specific Aim #1). Once optimized, these model systems will be used in studies intended to identify the mechanisms that permit allograft survival in tolerized mice (Specific Aim #2). In a final series of experiments, we will test the hypothesis that an optimized two element protocol will induce islet graft tolerance in spontaneously diabetic NOD mice (Specific Aim #3). Our immediate goal is to develop clinically applicable islet transplantation procedures that will eventually prove suitable for curing both auto-immune and non-autoimmune diabetes mellitus. It is our longer term hope that these procedures will be generally applicable to all transplant recipients who currently require lifelong generalized immunosuppression.